1. Field of the Invention
The present invention relates to a dielectric ceramic composition having a high dielectric constant and to a multilayered ceramic substrate produced by laminating and sintering an insulating ceramic material and a dielectric ceramic material.
2. Description of the Related Art
Performance of electronic parts in electronic fields is being significantly improved. In particular, recent trends in information apparatuses such as mainframe computers, mobile communication terminals and personal computers are improvements in processing speed and the development of miniaturized and multifunctional apparatuses. Such improvements in performance have been primarily achieved by increases in integration density, processing speed and function of semiconductor devices, as exemplified in VLSIs and ULSIs. Regardless of such improvements in semiconductor devices, the operation as a system is hindered by signal delay in substrates for connecting different devices, mismatch of cross-talk and impedance, and noise due to fluctuation of power voltage.
High-performance electronic parts for processing information at high speed are multi-chip modules (MCMs) in which high-performance semiconductor devices are mounted on a ceramic substrate. In multi-chip modules, multilayered ceramic substrates provided with three-dimensionally arranged line conductors are useful to enhance the mounted density of semiconductor devices such as LSIs and to secure sufficient conduction between LSIs. Alumina has been used as a conventional material for multilayered ceramic substrates.
Alumina, however, requires a high sintering temperature of at least 1,300xc2x0 C. When the multilayered ceramic substrate is produced by a co-sintering process, a high melting point metal, such as tungsten or molybdenum, must be used in line conductors for inner layers. Since these high melting point metals have a large specific resistance, they preclude high density wiring. In addition, aluminum has a large relative dielectric constant of approximately 10. When surface-mounted components such as semiconductor devices are operated at high speeds, significant signal delay will occur. Moreover, alumina has a larger thermal expansion coefficient than that of silicon used in semiconductors. Thus, heat cycles during use result in decreased reliability.
In order to solve these problems, low temperature sintering ceramic materials as composite materials composed of ceramic components and glass components have been intensively researched and have been employed in practice as multilayered ceramic substrates for multilayered modules and multilayered devices. A low temperature sintering ceramic material contains a ceramic composition as a matrix and a glass component. Since this material can be sintered at a low temperature, the versatility of possible design on properties and sintering temperature of the material is significantly high. Since the low temperature sintering ceramic material can be sintered with a low melting temperature metal having low specific resistance, such as silver or copper, the resulting multilayered ceramic substrate has superior high frequency characteristics.
In a recent trial for achieving further miniaturization of an overall module, passive devices such as capacitors and inductors, which are constituents of surface mounted devices (SMDs), are incorporated into a multilayered ceramic substrate. In such a case, the characteristics of the incorporated passive devices must be equivalent or superior to those of devices mounted on the substrate in order to achieve high overall performance of the module. Thus, materials for achieving high electrical characteristics of the surface mounted devices are generally selected as constituents of the multilayered ceramic substrate. For example, a dielectric ceramic layer having high dielectric constant is provided at portions for forming capacitors and an insulating ceramic substrate having a low dielectric constant, e.g., a low temperature sintering ceramic substrate, is provided at other portions in one embodiment.
A material having a specific dielectric constant Er of not greater than 10 is generally used in the insulating ceramic substrate having a low dielectric constant in order to suppress stray capacitance generating between mounted devices such as capacitors and inductors and coupling capacitance between lead lines, which stray capacitance and coupling capacitance causes deterioration of electrical characteristics. Furthermore, such a low dielectric constant is advantageous when the insulating ceramic substrate is used in high frequency regions.
Japanese Patent Publication No. Sho 56-82501, to the present assignee, discloses a dielectric ceramic material for microwave regions having a high dielectric constant and comprising a composition represented by the general formula:
xBaOxe2x80x94yTiO2-z(Nd1xe2x80x94mMem)O{fraction (3/2)}
wherein Me is a lanthanoid, 0xe2x89xa6mxe2x89xa61.0, and x, y and z have a molar ratio within a region surrounded by the coordinates A (x=0.20, y=0.70 and z=0.10), B (x=0.20, y=0.40 and z=0.40), C (x=0.02, y=0.70 and z=0.28) and D (x=0.02, y=0.40 and z=0.58) in a ternary diagram. This dielectric ceramic material has a high Q value and a significantly high dielectric constant of 2,000 or more. Moreover, the dependence of the electrostatic capacitance on temperature can be adequately adjusted by varying the content of the lanthanoid.
However, this dielectric ceramic material requires a high sintering temperature in a range of 1,300 to 1,400xc2x0 C. When a multilayered ceramic substrate is formed using the dielectric ceramic material, it is difficult to simultaneously sinter this dielectric ceramic material and a low melting point metal having low specific resistance, such as copper and silver. Moreover, this dielectric ceramic material has poor adhesiveness to insulating ceramic substrates and particularly low temperature sintering ceramic substrates. Thus, the resulting multilayered ceramic substrate will have low mechanical strength.
When a glass component is added in order to decrease the sintering temperature of the dielectric ceramic material, the mechanical strength of the substrate will be decreased compared to aluminum substrates or electric characteristics will be decreased depending on the type and the content of the glass component. When the mechanical strength of the substrate is maintained, the specific dielectric constant of the substrate is inevitably decreased. Thus, it is difficult to form capacitors having large capacity in the substrate. If such capacitors having large capacity are formed in the substrate, electrodes for the capacitors occupy a large area, which hinders miniaturization and high density integration of the substrate. On the other hand, a substrate having satisfactory electrical characteristics has low mechanical strength which is unsuitable for use in semiconductor devices.
Accordingly, it is an object of the present invention to provide a dielectric ceramic composition which can be sintered at a low temperature, has superior electrical characteristics and has high mechanical strength after sintering.
It is another object of the present invention to provide a multilayered ceramic substrate which can be sintered at a low temperature, has superior high-frequency characteristics and has high mechanical strength.
According to a first aspect of the present invention, a dielectric ceramic composition is produced by mixing and sintering a dielectric ceramic component represented by BaOxe2x80x94TiO2-(Nd1xe2x80x94mMem)O{fraction (3/2)} and a glass component comprising barium oxide, silicon oxide and boron oxide, wherein Me is a lanthanoid and 0xe2x89xa6mxe2x89xa61.0.
Preferably, the glass component comprises a mixture of about 20.0 to 65.0 mole percent of barium oxide, about 5.0 to 50.0 mole percent of silicon oxide and about 10.0 to 50.0 mole percent of boron oxide.
Preferably, the content of the glass component is about 3.0 to 35.0 percent by weight of the dielectric ceramic component.
In a preferred embodiment, when the dielectric ceramic component is that of the aforementioned Japanese Patent Publication No. Sho 56-82501 and is represented by xBaOxe2x80x94yTiO2-z(Nd1xe2x80x94mMem)O{fraction (3/2)}, x+y+z=1, x, y and z have a molar ratio within a region surrounded by the coordinates A (x=0.20, y=0.70 and z=0.10), B (x=0.20, y=0.40 and z=0.40), C (x=0.02, y=0.70 and z=0.28) and D (x=0.02, y=0.40 and z=0.58) in a ternary diagram.
The dielectric ceramic composition in accordance with the present invention further comprises lead oxide in an amount of about 17 percent by weight or less of the dielectric ceramic component.
The dielectric ceramic composition in accordance with the present invention has a high dielectric constant, a high Q value and superior temperature characteristics. In addition, the dielectric ceramic composition can be sintered at a low sintering temperature and has high mechanical strength.
According to a second aspect of the present invention, a multilayered ceramic substrate comprises an insulating ceramic layer having a low dielectric constant and a dielectric ceramic layer having a high dielectric constant, wherein the insulating ceramic layer is a low temperature sintering ceramic layer capable of being sintered at a low temperature, and the dielectric ceramic layer is produced by mixing and sintering a dielectric ceramic component represented by BaOxe2x80x94TiO2-(Nd1xe2x80x94mMem)O{fraction (3/2 )} and a glass component comprising barium oxide, silicon oxide and boron oxide, wherein Me is a lanthanoid and 0xe2x89xa6mxe2x89xa61.0.
Preferably, the glass component comprises a mixture of about 20.0 to 65.0 mole percent of barium oxide, about 5.0 to 50.0 mole percent of silicon oxide and about 10.0 to 50.0 mole percent of boron oxide.
Preferably, the content of the glass component is about 3.0 to 35.0 percent by weight of the dielectric ceramic component.
In a preferable embodiment of the multilayered ceramic substrate in accordance with the present invention, when the dielectric ceramic component is represented by xBaOxe2x80x94yTiO2-z(Nd1xe2x80x94mMem)O{fraction (3/2)}, x+y+z=1, x, y and z have a molar ratio within a region surrounded by the coordinates A (xxe2x88x920.20, y=0.70 and z=0.10), B (x=0.20, y=0.40 and z=0.40), C (x=0.02, y=0.70 and z=0.28) and D (x=0.02, y=0.40 and z=0.58) in a ternary diagram.
Preferably, the multilayered ceramic substrate in accordance with the present invention further comprises lead oxide in an amount of about 17 percent by weight or less of the dielectric ceramic component.
In a preferable embodiment, the low temperature sintering ceramic layer may be produced by sintering a glass ceramic composite material comprising barium oxide, aluminum oxide and silicon oxide.
Preferably, the multilayered ceramic substrate may be produced by laminating and sintering a green sheet for the insulating ceramic layer and a green sheet for the dielectric ceramic layer.
Alternatively, the multilayered ceramic substrate is produced by laminating and sintering a green sheet for the insulating ceramic layer and a thick printed film for the dielectric ceramic layer.
Preferably, the multilayered ceramic substrate in accordance with the present invention further comprises a copper-based conductive pattern.
In the multilayered ceramic substrate in accordance with the present invention, the bonding strength between the insulating ceramic layer and the dielectric ceramic layer is high. Moreover, the dielectric ceramic layer has a high dielectric constant, a high Q value and superior temperature characteristics. Thus, the multilayered ceramic substrate can be sintered at a low temperature and has superior high-frequency characteristics and high mechanical strength.